1. Field of the Invention
This invention relates to a silicone composition which is capable of yielding a cured product having good thermal conductivity and is thus useful as a protective sealing material for electric and electronic parts such as IC.
2. Description of the Prior Art
Silicone compositions of the addition curing type which are composed of vinyl group-containing organopolysiloxanes and organohydrogenpolysiloxanes having hydrogen atoms joined to the silicon atoms are able to form a cured product of a gel-like elastomer or a so-called silicone gel. This type of silicone composition has wide utility in the field of protective sealants for electric and electronic parts such as integrated circuits.
However, the known silicone composition is so small in thermal conductivity that when it is used for sealing of electric elements of a large capacitance, the heat generated from the element cannot be dissipated to a satisfactory extent. This eventually leads to the problem that the influence of the heat on a solder or the element cannot be suppressed as desired.
It is known that in order to improve the thermal conductivity of the silicone gel, use of fillers with good thermal conductivity is effective. Such fillers include, for example, silica powder, aluminium oxide powder, silicon carbide powder, silicon nitride powder, aluminium nitride powder, magnesium oxide powder, diamond powder, powders of metals such as iron, stainless steels, copper and the like and carbon powder.
Of these fillers, the metallic powders and carbon powder are electrically conductive and cannot be used in the IC-sealing silicone compositions which have to be electrically insulating in nature.
Although the silicon nitride, aluminium nitride and magnesium nitride powders may be used from the standpoint of electric insulation, all of them are hydrolyzable, with the attendant problem that they are short of long-term stability. These powders may be coated on the surfaces thereof with non-hydrolyzable materials so as to avoid the above problem. However, such a coating is not good in economy.
The silicon carbide and diamond powders have so high a hardness that it is not convenient to use them because of wear or break down of wirings of elements by means of the exposed edges of the powder. In addition, the diamond powder is too expensive.
In view of the above, fillers which are usable in practical applications include silica and aluminium oxide powders. However, the silica powder is not satisfactory with respect to the thermal conductivity. In order to impart high thermal conductivity to the silicone composition by the use of the silica powder, workability will be greatly impeded in view of the increase in viscosity of the resultant silicone composition. With aluminium oxide powder, a vital disadvantage is produced in that elements to be protected undergo corrosion.